Does FOX NOW support Apple TV Single Sign-On?

Yes, FOX NOW supports single sign-on for participating TV providers on Apple TV devices (4th generation or later). If you have already signed-in to another app with your TV provider login credentials, you may automatically sign in to FOX NOW using the same single sign-on login credentials. Though you only need to enter your credentials once, you must grant each app, including FOX NOW, permission to access your TV provider account. If FOX NOW is the first app with which you sign-in using your TV provider credentials, you may use the same credentials to automatically sign in to other apps supported by Apple & participating TV providers offering single sign-on.

You can sign-in with only one TV provider at a time on Apple TV devices (4th-generation or later).